Don't commit mortgage fraud!
If you were going to commit mortgage fraud a couple of years ago, it was quite tricky, and really needed to be something of a team effort. For self-employed applicants, the lender would generally want to get a letter from an accountant, confirming that the income that the applicant had said they had was an accurate reflection of their actual income. If you'd exaggerated your income to the lender, you'd therefore need a dodgy accountant at least, and you might want a dodgy mortgage broker too (from time to time brokers and accountants did get into trouble for backing up spurious applications, sometimes on a fairly prolific basis).
Mortgage companies have now become less interested in accountants' letters, and obsessed instead with an HMRC document called an SA302. An SA302 is issued by HMRC and shows the income someone put on their tax return. HMRC aren't verifying that the income on the tax return is right in any way, they're just saying that's what was on the return. The lenders though think that this third party document is more robust than a letter from an accountant.
So, it's now miles easier for a self-employed applicant to exaggerate their income and get a mortgage that their income doesn't justify, and they can do it all alone without having to find dodgy professionals to help them out. All you'd have to do is a) file your own tax return, with your income exaggerated, b) get the SA302 from HMRC and use it to get your mortgage and then c) go back to HMRC to correct your tax return ("I forgot to put all my costs on the return! What a fool I am! Sorry.") before the tax payment becomes due.
So, a measure that's supposed to reduce the possibility of mortgage fraud actually makes it much easier, because the lenders don't really understand what an SA302 is and the lack of assurance it offers.
Don't commit mortgage fraud though.